Agilent Technologies 7000 Triple Quadrupole GC/MS, Maintenance Manual

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156 Troubleshooting and Maintenance Manual
7
Analyzers and Collision Cell
Quadrupole Mass Filters
The mass filters separate ions according to their mass-to-charge ratio ( m/z ). At
a given time, only ions of a selected mass-to-charge ratio can pass through the
filter to the detector. The mass filters in the MS are quadrupoles (
Figure 30 ).
The quadrupole is a set of four fused-silica (quartz) rods coated with a thin
layer of gold. The four hyperbolic surfaces create the complex electric fields
necessary for mass selection. Opposing rods are connected; adjacent rods are
electrically isolated. One pair has positive voltages applied, the other negative.
A combined direct current (DC) and radio frequency (RF) signal is applied to
the two pairs of rods. The magnitude of the RF voltage determines the
mass-to-charge ratio of the ions that pass through the mass filter and reach
the detector. The ratio of DC-to-RF determines the resolution (widths of the
mass peaks). There are several parameters that control the DC and RF
voltages. All these parameters are set by Autotune, but also can be manually
adjusted in the Edit Parameters window:
• Width gain
• Width offset
• DC polarity
• Mass axis gain
• Mass axis offset